1. Field of the Invention
This invention relates to a wax hydrofining process. More particularly, this invention relates to a process for hydrofining high melting microwax stocks such as deasphalted tank bottoms wax over a nickel or cobalt molybdate on alumina catalyst in two successive stages, with the temperature in the second stage not exceeding about 550.degree. F. and being maintained at least about 50.degree. F. lower than the temperature in the first stage, recycling the hydrofined wax product back into the second stage, and periodically increasing the temperature in the first stage as the catalyst ages.
2. Description of the Prior Art
Microcrystalline wax is used in the manufacture of many products such as food containers, waxed papers, coating materials, electrical insulators, candles, etc., and is usually obtained from the highest boiling fraction of a crude oil. In the manufacture of conventional microcrystalline waxes, the bottoms stream from a vacuum pipestill is deasphalted to produce a heavy deasphalted oil which is then extracted to partially remove aromatics therefrom. The term microcrystalline wax generally refers to deoiled (to less than about 5 wt.% oil) wax having a melting point varying from about 140.degree. to 180.degree. F. which is recovered from this deasphalted, extracted oil by dewaxing and deoiling. The term petrolatum applies to the non-deoiled residual microwax having an oil content of 30 to 40%.
The wax recovered from the lower boiling vacuum pipestill side streams is generaly termed paraffin wax. Both deoiled residual microwax and petrolatum are much more difficult to refine than paraffin wax because they are higher in molecular weight and contain many more cyclic structures particularly aromatics and fewer straight chain n-paraffins. The raw microwax obtained by such a process is characterized by a poor odor, a dark color and it contains aromatic impurities as shown by ultraviolet absorption tests. Thus the microwax must be furthe refined in order to obtain useful products therefrom. In the past microwax was contacted with solid absorbent materials such as bauxite or clay to absorb the aromatic compounds therefrom which impart the unfavorable properties to the wax.
Various improvements in the refining of microcrystalline waxes have been made over the years, most notable of which have been those directed towards catalytic refining of the wax in the presence of hydrogen, also known as hydrofining. For example, U.S. Pat. No. 3,052,622 discloses taking a crude oil residua and simultaneously deasphalting and extracting the aromatics from it via the Duo-Sol process to obtain a waxy petroleum residue which is then hydrofined by passing the wax, in the presence of hydrogen, over a catalyst of nickel oxide on bauxite. The hydrofined product is then dewaxed via a conventional solvent dewaxing process using toluene and MEK as the dewaxing solvents. U.S. Pat. No. 3,022,245 discloses a relatively low pressure process for hydrofining paraffin waxes having melting points of from about 115.degree. to 155.degree. F., by using a cobalt molybdate on alumina catalyst and treating the wax in two stages at relatively low pressures of less than 1000 psig with the temperature in the second stage maintained about 100.degree. F. lower than the temperature in the first stage. U.S. Pat. No. 3,275,545 discloses a low pressure, single stage process for hydrofining conventional microwaxes using catalysts containing from 2 to 5 wt.% cobalt oxide and 8 to 15 wt.% molybdenum oxide and recycling a portion of the hydrofined product back into the inlet of the hydrofining zone in order to maintain the concentration of multi-ring aromatic hydrocarbons therein below about 4% by weight thereby avoiding deactivation of the catalyst. Preferred pressures in the hydrofining zone are disclosed as varying between about 500 and 1000 psig. U.S. Pat. No. 3,365,385 discloses another low pressure process for hydrofining microwax using a cobalt molybdate catalyst wherein the pressure in the hydrofining zone is maintained below 900 psig. Critical features are the use of low space velocities and a liquid phase reaction wherein the catalyst is kept submerged in liquid feed.
However, none of the prior art processes for hydrofining microcrystalline waxes have been found satisfactory for use with tank bottoms wax. Tank bottoms wax differs from ordinary microcrystalline waxes in that it is derived from the whole crude oil. That is, crude oil is pumped out of the ground and transferred to storage tanks in the oil fields and then sent to refineries. In the storage tanks, a relatively high boiling, high melting point microcrystalline wax slowly precipitates from the crude oil as same cools off in the tanks and deposits or plates out on the bottom and on the walls inside the tanks. This wax is referred to as tank bottoms wax. Periodically, the tanks must be completely drained and scraping equipment is used to scrape out the wax, from whence is derived the term tank bottoms wax. The melting point of this wax is higher than ordinary microcrystalline wax in that it is usually at least about 190.degree. F. and contains a substantially higher amount of aromatic and naphthenic and some asphaltic constituents. In addition, tank bottoms wax contains high boiling components which are not in ordinary residual microwax. These composition differences explain the high hardness and melting point of tank bottoms wax and its refractory nature in that it is more difficult to hydrofine because of its higher aromatics content. This wax is presently refined by deasphalting, deoiling and then contacting with bauxite or other solid absorbent in order to remove the aromatics therefrom and then acid treating to produce a refined wax of good color, odor and color stability that can be used for various purposes which require meeting FDA requirements such as for food container linings, etc. It would be a useful contribution to the art therefore if a way could be found to hydrofine this tank bottoms wax instead of having to use the rather expensive and cumbersome solid adsorption methods.